Emergency power systems for electromagnets have been available to insure that the magnets do not drop the load which they are holding in the event of a power failure. Typically, these systems utilize a bank of batteries which are charged directly off of the incoming DC power line, and thus, cannot be charged to a value in excess of the incoming power line voltage. When a fault occurs, a switching arrangement is provided to introduce these batteries into the circuit in order to provide sufficient voltage to the magnet so that it does not drop its load. However, inasmuch as these batteries are initially at a voltage which approximates the voltage of the incoming power source and subsequently decay therefrom, it is possible that the batteries will not provide sufficient voltage to the electromagnet in order to hold the load. In essence, in this arrangement the voltage applied to the electromagnet by the battery bank is always less than the voltage which would be applied by the main power source, and this can present problems in holding a difficult load such as a multiple plate load wherein the flux in the last plate is not strongly coupled to that in other plates comprising the load. This condition could result in the electromagnet starting to drop all or a portion of the load.
If the batteries are charged from a source other than the incoming DC power line, they can be charged to a voltage in excess of the incoming power line voltage, however, these systems typically utilize mechanical transfer means to connect the batteries to the magnet in the event of a power failure. Because mechanical transfer means are used, the transfer is not instantaneous and thus the magnetic flux, particularly the flux associated with relatively small magnets, could be affected resulting in the possible dropping of the load.
Because of the foregoing, it has become desirable to develop a system which always applies sufficient voltage to an electromagnet to hold its load and which instantaneously transfers to the battery supply in the event of a main power source failure.